Window of Opportunity: PNC’s Passive Ventilation

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It can take a large HVAC system to keep a commercial building well-ventilated and comfortable. The recently completed Tower at PNC Plaza decided to incorporate the old-fashioned way of getting fresh air: opening a window. Rising 33 stories above Pittsburgh, the Tower has an envelope that can ventilate as well as insulate. The team that handpicked the novel elements of the tower gave us the details behind the design at our Skyscraper that Breathes panel last month. Doug Gensler (Gensler), described how the vision for the tower came together. Denzil Gallagher, (BuroHappold Engineering), explained the integrated systems that make the building work. Tom Paladino (Paladino & Co.), the project’s sustainability consultant, emphasized the structure’s strong “double bottom line.”

To scout out proven successes in sustainable skyscrapers, Gensler and his team toured projects in six cities outside the US. The inspiration for a double-skin façade and passive radiant system came from multiple European buildings. Another notable inspiration was Canada’s Manitoba Hydro building, which provides fresh air year-round thanks to a prevailing southerly wind that blows 365 days a year. But back in Pittsburgh, seasonal variations mean fresh air is only practical 40% of the year, in the spring and fall. The team therefore decided to implement a combined natural and mechanical system for ventilation. During temperate months, “popper” windows in the façade open and air infiltrates the building, drawn by the stack effect of a solar chimney. The HVAC motors turn off and the building’s energy use drops by half. In the winter and summer, the windows are closed and the envelope reverts to being a buffer against outside extremes. While fresh air is only available seasonally, Gensler’s team designed for daylight to penetrate as deeply as possible throughout the year.

Gallagher and his firm analyzed what technologies did, and didn’t, make sense for the Tower using an arsenal of tools that included daylight analysis, energy modeling, 3D Building Information Modeling, and computational fluid dynamics. Still, it wasn’t enough—Gallagher wanted systems that were tried and tested. So, physical mockups of two of the building’s main features were made: the envelope and the solar slab on the roof. While the mockups weren’t cheap, they verified that systems would work, providing lessons learned and time saved on the actual construction. Denzil pointed out that the width of the double skin varies based on the building’s orientation. Thirty inches between the two skins around much of the circumference allows for dynamic management of ventilation. On the western side a much narrower gap allowed shading between the two walls to cut down on solar glare and heat gain. On the roof heat gain is harnessed by the building’s solar chimney. An angled greenhouse encloses a slab that sits atop two shafts that run through the building. When the slab heats up, stack effect is enhanced and motorized dampers regulate how much air is drawn up through the building based on seasonal needs.

What more could you want from a building that cuts energy use and costs in half? Increased revenue, of course. Paladino noted that PNC already knew the bank’s “green” branches outperformed the company’s conventional brick buildings in terms of employee engagement. The same philosophy applied to the tower. He cited two studies: one showing that daylight increased productivity by 5.5% and another where natural ventilation bumped productivity by 8.5%. (Not to mention Harvard’s recent study that shows better ventilation can double cognitive performance.) With daylight penetrating 91% of the building’s floor area and fresh air in the spring and fall, the building has a lot to offer tenants, and educating staff on the building’s systems and advantages was an important part of initial occupancy. Town halls were held after construction was completed to provide introductory knowledge that is reinforced daily via a “Beacon” in the lobby that displays building’s energy use is for all to see.